This application is related to Japanese Patent Applications No. Hei 11(1999)-260742 filed on Sep. 14, 1999 and 2000-254017 filed on Aug. 24, 2000, whose priorities are claimed under 35 USC xc2xa7119, the disclosure of which is incorporated by reference in its entirety.
1. Field of the Invention
The present invention relates to an apparatus for producing polycrystalline silicon sheets which are materials for silicon wafers usable for solar cells and others and a production method using the apparatus.
2. Description of Related Art
Polycrystalline silicon wafers to be used for solar cells and the like are produced, for example, by melting with heat a high-purity silicon material to which a dopant such as phosphorus, boron or the like is added, in an inert atmosphere in a crucible, casting the resulting silicon melt (referred to as melt for short hereinafter) into a mold, gradually cooling the melt to obtain a polycrystalline ingot and slicing the ingot with a wire saw or an inner diameter blade, thereby obtaining desired wafers (see Japanese Unexamined Patent Publication No. HEI 6(1994)-64913). Since this method requires a slicing process, slicing loss corresponding to the thickness of a wire or an inner diameter blade occurs. That decreases the yield of wafers and as a result it is impossible to supply the wafer at low prices.
Also disclosed is a method for producing silicon sheets without need of slicing the ingot (see Japanese Unexamined Patent Publication No. HEI 7(1995)-256624). According to this method, silicon melt is fed into a heating horizontal mold. A carbon plate is moved in a horizontal direction and contacted directly to the melt. When the melt is stuck to the plate, the plate is drawn away horizontally and cooled by cooling gas blown by a gas blow portion of a cooling system. Thus silicon sheets are continuously obtained. According to this method, since the thickness of silicon sheets is controlled by a thickness control plate, it is impossible to control the thickness to 400 xcexcm or less, which is required for solar cells.
Japanese Unexamined Patent Publication No. HEI 10(1998)-29895 discloses an apparatus and a method for producing polycrystalline silicon sheets by which silicon melt obtained by melting a silicon material by a heater in a crucible is contacted to a rotary cooling member of a heat-resistant material having a rotary shaft parallel to the surface of the melt, while cooling the rotary cooling member with liquefied nitrogen gas, so as to grow silicon crystals on the surface of the rotary cooling member, and the silicon crystals are stripped from the rotary cooling member to give a polycrystalline silicon sheet.
Crystals of polycrystalline silicon sheets produced by the apparatus and method disclosed by the above-mentioned Japanese Unexamined Patent Publication No. HEI 10(1998)-29895 grow in columnar form but the size of the crystals is not precisely controlled. Therefore, it is necessary to pay the closest attention to the control of temperature distribution of the melt, rotation speed of the rotary cooling member and others for maintaining high quality of polycrystalline silicon sheets for solar cells. If these conditions change even slightly, the crystals growing on the surface of the cooling member exhibit a dendrite structure of tree branch-like elongate crystals, which are not suitable for the solar cells.
The above-mentioned Japanese Unexamined Patent Publication No. HEI 10(1998)-29895 discloses in its claim 3 an apparatus for producing polycrystalline silicon sheets which forms polycrystalline silicon sheets on the surface of a rotary cooling member coated with a ceramic sheet such as boron nitride by contacting the rotary cooling member with silicon melt. However, since the surface of the rotary cooling member of this apparatus is entirely covered with the ceramic film, the entire surface of the rotary cooling member serves as a nuclei for crystal growth. Accordingly, the polycrystalline silicon sheets cannot be easily stripped from the surface of the rotary cooling member, that is, a sufficient stripping property cannot be obtained, when the sheets are continuously taken away from the surface of the rotary cooling member.
In view of the above-mentioned problems, an object of the present invention is to provide an apparatus and a method for producing polycrystalline silicon sheets which allow silicon crystals to grow in good form and also provide easy stripping of the polycrystalline silicon sheets from the surface of a cooling member.
The present invention provides an apparatus for producing a polycrystalline silicon sheet comprising a crucible; a heating unit for heating a starting material of silicon fed in the crucible; and a cooling unit for contacting a melt of the starting material melted by heating to a cooling face of a cooling member, thereby obtaining a polycrystalline silicon sheet in which crystals of silicon are grown, wherein the cooling face of the cooling member has a sheet adhering portion for providing a silicon starting point of crystallization (i.e., crystallization growth point) and allowing adhesion of the polycrystalline silicon sheet of grown crystals and a sheet stripping portion for allowing easy stripping of the polycrystalline silicon sheet.
The sheet adhering portion formed on the cooling face serves as the starting point of crystallization for allowing growth of silicon crystals and adhesion of the polycrystalline silicon sheet, while the polycrystalline silicon sheet adheres only weakly or does not adhere substantially to the sheet stripping portion provided in the rest of the cooling face. Thus it is possible to attain both the adhesion property and the easy stripping property for the polycrystalline sheet which forms as the crystals grow.
In other words, by arranging the sheet stripping portion, which has so weak adhesive force to the sheet that it cannot be a substantial sheet adhering member by itself, and the sheet adhering portion, which has a strong adhesive force to the sheet and can compensate for the weak adhesive strength of the sheet stripping portion to the sheet or is responsible for the adhesion to the sheet, in an appropriate relationship, it is possible to obtain a polycrystalline silicon sheet having a desired crystal form growing from the sheet adhering portion as the starting point of crystallization as well as to strip the obtained sheet easily without need to use a stripping agent as conventionally used.
Difference in the adhesive force to the polycrystalline silicon sheet between the sheet adhering portion and the sheet stripping portion is based on their different shapes and/or materials. An optimal difference in the adhesive force can be established for acquiring both good adhesion and easy stripping of the sheet by controlling conditions such as the temperature of the starting material melt when the melt is contacted to the cooling face to grow silicon crystals, the temperature of the cooling face, contact time and the like.
In the polycrystalline silicon sheet produced by the apparatus for producing polycrystalline silicon sheets of the present invention, the silicon crystals grow in a columnar form from the sheet adhering portion as the starting point of crystallization. Thus it is possible to obtain polycrystalline silicon sheets suitable for solar cells and the like.
Furthermore, since the resulting polycrystalline silicon sheet can be easily stripped from the cooling face without use of a stripping agent, it is possible to produce polycrystalline silicon sheets continuously.
The sheet stripping portion in the present invention is formed of a material having a relatively weak adhesive force to the polycrystalline silicon sheets by itself. Such a material may preferably be carbon because it is heat resistant and it does not inhibit crystallization of silicon.
The sheet adhering portion in the present invention is different in shape and/or material from the sheet stripping portion. As for its shape, the sheet adhering portion may have dots arranged regularly or random on the cooling face. Examples of such dots include a plurality of columnar or polygonal studs having flat or curved heads or sides to provide small areas for growing crystals on surfaces of the studs as starting points of crystallization.
The heads or sides of the studs arranged in dots serve as the starting points of crystallization and allow adhesion of the formed polycrystalline silicon sheet.
The sheet adhering portion may have a surface area occupying preferably 0.1 to 25%, more preferably 1 to 10%, of the cooling face. The surface area of the above-mentioned stud may mean the surface area of its head, i.e., the area of an image of the stud projected on the cooling face, since the surface area of the sides of the stud is negligible as compare with that of the head of the stud. If the surface area of the sheet adhering portion is smaller than 0.1% the adhesive force of the polycrystalline silicon sheet is not enough, and if its surface area is larger than 25%, the polycrystalline silicon sheet may be damaged when it is stripped from the sheet adhering portion.
The above-mentioned dots of the sheet adhering portion may have a diameter of 1 to 500 xcexcm and a height of 4 to 100 xcexcm, for example, if they have a columnar shape.
The material for the dots is not particularly limited so long as it exhibits heat resistance at temperatures higher than the melting point of silicon and it can be easily made in dots on the cooling face, but it may preferably be a ceramic since ceramic can be easily shaped on carbon. Carbon is typically used for the cooling face. The ceramic may contain any one of silicon carbide, silicon nitride and boron nitride, for example.
In the present invention, the sheet adhering portion is preferably comprised of ceramic dots arranged on the cooling face. This is because ceramics are considered to be able to provide starting point of crystallizations suitable for growth of silicon crystals.
The cooling unit may have the cooling face in part of its flat face or may be a rotary cylinder having the cooling face on its periphery. If the cooling unit is the latter, i.e., a rotary cylinder, polycrystalline silicon sheets can be continuously produced.
In another aspect of the present invention, the cooling member may include a plurality of cooling member pieces each having a flat cooling face, the cooling member pieces being connected to each other in a caterpillar form, and the caterpillar-formed cooling member pieces revolves in an up-and down direction with respect to the crucible in such a manner that one to fifty of the cooling faces of the revolving cooling member pieces contact the melt sequentially. With this construction, the apparatus of the present invention will be more suitable for mass production because polycrystalline silicon sheets can be continuously produced. The apparatus for producing polycrystalline silicon sheets having this type of cooling unit may further include a sheet stripping mechanism provided with at least one suction case having at a distal end a porous surface disposed to face the cooling faces of the revolving cooling member pieces and with a vacuum pump for, when the suction case faces the cooling face of the cooling member piece to which the polycrystalline silicon sheet adheres, sucking the inside of the suction case from a proximal side to take the polycrystalline silicon sheet onto the porous surface by suction. With this construction, more efficient mass production can be realized.
These and other objects of the present application will become more readily apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.